The present invention relates to the field of semiconductor devices and their manufacture. More specifically, the present invention relates to the field of semiconductor devices which have multiple vertically stacked conducting layers and where contact needs to be made to two or more of these conducting layers.
Nearly all semiconductor devices comprise a plurality of vertically stacked layers. In some devices, for example, a field effect transistor (FET), there is a need to measure transport laterally through an active layer and to modulate this transport by applying a field using a further conductive layer or region within the device. To make contact to the active layer, a source contact and a drain contact are used.
If the further conductive layer or region is a semiconductor layer or region within the device, there is a problem in that the source and drain contacts can also connect to the contact region thus causing the device to short through the lower contact region.
Many solutions to this problem have been proposed, for example, the use of shallow ohmic contacts whose penetration can be controlled so that they only penetrate to the active layer. These contacts have the problem in that their penetration can be difficult to control and even if it if correctly controlled they still have a tendency to leak to the lower contact region. To minimise the problems of the contacts leaking or shorting, a relatively large distance is introduced between the active layer and the lower contact region, thus increasing the size of the device. In addition, a large operating voltage will be required that inevitably leads to other problems e.g. current leakage and heating.
Another conventional method is to use a conducting substrate as its lower contact region. However, this technique often still require the use of shallow ohmic contacts and hence the associated problems. Moreover, it is very difficult to align this ohmic contact on the back of the wafer substrate region with the active area of the device on the wafer surface.
Other techniques have involved complicated regrowth techniques such as those described in GB 2305003 or using highly specialized equipment such as in situ focused ion beam patterning (Linfield et al, Semicond Sci Tech vol. 8, pages 415-422 1993).
Previously, suspended structures have been used to test hypothesis, e.g. Blick et al. Phys. Rev. 62, pages 17103 to 17107 (2000) where a two dimensional electron gas (2DEG) is suspended in order to study both electrons and phonons. Other suspended structures, e.g. Itskevich et al, Appl. Phys. Lett. 76, pages 3932 to 3934 (2000) have not used a suspended structure to obtain lateral transport through a low dimensional carrier gas.